Transformation induced plasticity (TRIP) steel sheet is one that can achieve improved strength and ductility as a meta-stable austenitic structure remaining in the steel sheet is transformed by a plastic deformation applied from outside.
For a typical steel sheet, as the strength increases, the elongation is decreased, and vice versa. However, the TRIP steel sheet has good strength as well as good elongation.
The metastable austenitic structure necessary for the TRIP steel sheet is formed as follows: a steel sheet whose basic structure is formed of a ferrite and a pearlite with a suitable amount of Mn and Si is maintained at an appropriate temperature between Ac1 and Ac3 at which the ferrite and austenite structures coexist, allowing austenite stabilizing elements, particularly carbon, in the steel sheet to form a solid solution in the austenite. It is then quenched in bainite transformation region having a temperature lower than pearlite transformation region, then subjected to a transformation process at a constant temperature for several minutes, to form a pro-eutectoid ferrite in the austenite structure, which allows carbon to diffuse from the ferrite structure to the austenite structure and as a result carbon concentration is increased. In this way, the transformation starting temperature of the austenite to martensite, i.e., Ms point, can be lowered to below room temperature. Thus, even at the room temperature, the austenite may remain stable without being transformed to martensite. When the steel sheet containing this residual austenite undergoes a plastic deformation, the plastic deformation functions as a mechanical driving force for the residual austenite to transform to the martensite, and such martensite transformation makes a processing cure rate to increase, which in turn delaying necking, and as a result the ductility increases with the strength.
In the related prior art, Korean Laid-open Patent Publication No. 2011-0100868 (Publication Date: Sep. 15, 2011) discloses a high-strength cold-rolled steel with excellent tensile strength, yield strength and elongation, and a method of preparing the same.